1. Field of the Invention
This invention involves the fabrication of optical fibers.
2. Description of the Prior Art
During the past decade, optical fiber fabrication technology has advanced to the point where fibers with losses on the order of one db per kilometer may be fabricated as a matter of course. Low loss fibers (less than 10 db/km at 825 nm.) have made optical communications an economically viable reality.
Two major optical fiber structures are most prevalent. The first involves what is commonly referred to as a single mode fiber. This fiber has a relatively small diameter core region, and a relatively large diameter cladding region of lower index of refraction. The dimensional and physical characteristics of the core region results in the transmission of radiation in only one electromagnetic configuration, or mode. In this single mode configuration, large amounts of energy are transmitted in the cladding and hence its purity is of significant concern.
The second major optical fiber structure is commonly referred to as the multi-mode configuration. In this configuration, the core region is of a size on the same order of magnitude as the cladding. Such a fiber can support numerous optical modes. Since the velocity of the light within the fiber varies from mode to mode, a single pulse transmitted in such a fiber suffers significant temporal distortion, referred to as mode dispersion. Such mode dispersion is minimized by, for example, fabricating the fiber with a radial gradation in the index of refraction--the larger values of index of refraction appearing near the center of the core. In this graded configuration, only a minimal amount of electromagnetic energy is transmitted in the cladding. Nevertheless, the purity of the cladding is of concern, especially for very low loss fibers.
There are two basic processing techniques which have been developed and refined to the point where they can regularly produce low loss fibers. Each of them involves the fabrication of a relatively large structure--an optical fiber preform--from which the optical fiber is drawn. The index of refraction characteristics of the preform are identical to those of the ultimate fiber.
The first fabrication process is commonly referred to as the "soot" process and is described in U.S. Pat. Ser. Nos. 3,775,075 and 3,826,560 assigned to the Corning Glass Works and hereby incorporated by reference into this application. In this process, translucent glass precursor particulates are deposited on a rod-like mandril by a deposition device which traverses the rod longitudinally numerous times. Before each pass, the dopant concentration in the precursor vapor may be changed so that a structure with an appropriate refractive index gradation will be ultimately formed. Susequent to deposition, the rod may be removed, and the glass precursor structure is "consolidated" by heating in an appropriate environment to yield a transparent glass optical fiber preform from which the optical fiber is drawn. In this technique, the glass precursor particulate deposition device is most often a hydrolysis burner.
An advantage of this "soot" technique is its relatively rapid fabrication rate or throughput. A disadvantage stems from the chemistry inherent in the exemplary hydrolysis deposition device. Water vapor which is produced during the hydrolysis is incorporated into the fiber and becomes a source of significant insertion loss. Special procedures may be followed to minimize this effect, but it remains a source of serious concern.
The second prevalent fiber fabrication process is referred to as the modified chemical vapor deposition process (MCVD), described in U.S. patent application Ser. No. 828,617 and hereby incorporated by reference into this application. In this process, appropriate glass precursor reactants are flowed through a tubular starting member. The tube is heated by an external heat source which periodically traverses the tube. When the reactants pass the hot zone generated by the traversing heat source, they react, primarily homogeneously--i.e., in the center of the tube away from the wall--to yield translucent glass precursor particulates. These particulates then deposit downstream of the heat source under the thermophoretic influence of a favorable temperature gradient and are consolidated into a transparent glass by the passing hot zone. In this manner, numerous layers may be deposited and appropriate index of refraction configurations formed. The starting member may be used either as an optically active cladding (i.e., participating in the transmission process) or as an inactive jacket. In the latter case, the cladding may be deposited on the interior of the tube prior to deposition of the core. Barrier layers to prevent migration of impurities from the tubular starting member to the deposited material may also be deposited. However, in the prior art practice of MCVD, the cladding was completed prior to deposition of the core. Subsequent to deposition, but prior to, or simultaneously with, drawing the structure may be collapsed to a solid optical fiber preform.
A primary advantage of the modified chemical vapor deposition process lies in the fact that the glass precursor particulates are formed in a reaction zone which is devoid of hydrogen bearing compounds and protected from externaly born contaminants. Consequently, an ultra-pure material results with little trace of deleterious water or other contamination.